Embodiments of the present invention relate to a chemical mechanical polishing pad and related methods and apparatus.
Chemical mechanical planarization (CMP) is used to planarize the surface of a substrate, in the manufacture of the integrated circuits and displays. A typical CMP apparatus comprises a polishing head that oscillates and presses a substrate and polishing pad against one another, while a slurry of abrasive particle is supplied therebetween. CMP can be used to planarize the surfaces of dielectric layers, deep or shallow trenches filled with polysilicon or silicon oxide, metal films, and other such layers. It is believed that CMP polishing typically occurs as a result of both chemical and mechanical effects, for example, a chemically altered layer is repeatedly formed at the surface of the material being polished and then polished away. For instance, in the polishing of metal features or layers, a metal oxide layer is formed and then removed repeatedly from the surface of the metal being polished.
To control slurry distribution, the polishing pad surface typically has a pattern of perforations or grooves to control the distribution of polishing slurry across the substrate. CMP polishing results depend upon the chemical and mechanical interaction of the polishing surface of the polishing pad which is pressed against the substrate the polishing pad, the abrasive particles of the polishing slurry, and the reactive material of the substrate. A non-uniform distribution of polishing slurry across the substrate surface can result in uneven polishing of the substrate surface. Thus, it is desirable to have a polishing surface of the polishing pad capable of providing a uniform distribution of slurry across the substrate surface.
Several pad designs have been developed to provide more uniform polishing slurry distribution across the surface of the substrate. One pad design uses concentric circular grooves or spiral grooves, as for example, disclosed in commonly assigned U.S. Pat. No. 5,984,769 which is incorporated herein by reference in its entirety. The circular grooves fill with polishing slurry during the polishing process to maintain a more uniform distribution of polishing slurry across the substrate surface. While such pad designs improve overall polishing uniformity, they also tend to trap slurry in predefined regions of the polishing surface of the pad resulting in excessive polishing of corresponding substrate regions. Also, because the slurry is trapped in a closed circular groove, the polishing slurry is prevented from continuously flowing from the center of the pad to its outer edge, which is desirable to remove polishing byproduct and worn slurry particles. In another pad design, an X-Y grooving pattern is provided on the polishing surface with different channel lengths. However, when the polishing pad and substrate oscillated with a rotating motion, the X-Y pattern generates a polishing slurry flow imbalance due to the axial symmetry of the groove pattern, and can also result in slurry being rapidly ejected from the edge of the pad surface.
A further problem with conventional designs arises because the pad has to be both sufficiently rigid to planarize the substrate surface and sufficiently compliant to press the polishing pad with uniform pressure against the substrate surface. To properly planarize the substrate, the polishing pad should polish only the peaks and not the valleys of the surface topography of the substrate. However, if the polishing pad is too easily compressed under localized stresses applied at pad regions which are directly above peaks in the substrate topography, the substrate region that surrounds the peak becomes excessively polished, which is undesirable. The pad has to be sufficiently rigid so that it does not compress too much under the load applied by the topographic peaks on the substrate, and yet sufficiently flexible to conform to, and uniformly polish, a slightly warped substrate.
To address the simultaneous flexibility and rigidity requirements, polishing pads are typically fabricated with two stacked layers of different materials, the bottom layer being made of a compliant springy material and the top layer being made of a rigid material that serves as the polishing surface. However, in use, polishing slurry tends to wick into the interface between the two layers starting from the outer peripheral edge of a layer toward the center of the two layers. This wicking can cause undesirable changes in the compressibility of the compliant spring layer. Excessive wicking can also cause polishing slurry to penetrate deep enough between the layers to reach and change optical properties of a pad window in the pad. It is desirable to have a polishing pad that is compliant and springy as well as sufficiently rigid to serve as a polishing surface.
Accordingly, it is desirable to have a polishing pad with a polishing surface that provides uniform and repeatable planarization of substrates. It is further desirable to have patterned features on the polishing surface of the polishing pad that cause the slurry to be uniformly distributed across the substrate surface. It is further desirable to have a polishing pad that is compliant while still providing a substantially rigid polishing surface.